1. Field of the Invention
The present invention relates to a novel magnetic coating material which is prepared by dispersing a fine powder of a ferromagnetic material in a binding material of a macromolecular compound comprising an epoxy group in it, followed by adding a silane compound comprising an amino group or a thiol group in it to the dispersion and to a novel magnetic recording medium which comprises a magnetic layer prepared from the magnetic coating material.
2. Prior Art
Higher recording density is an increasingly intensive requirement for magnetic recording media. In the case of recording tapes, for example, it is required that higher saturated magnetic flux density, higher rectangular ratio Rs which is obtained by dividing residual magnetic flux density Br by saturated magnetic flux density Bm, lower roughness of surface, excellent flexibility and excellent wear resistance are achieved simultaneously. When very fine powder of ferromagnetic materials having a specific surface area in the range from 30 m.sup.2 /g to 70 m.sup.2 /g is utilized because of the requirement for a higher S/N ratio and other properties as mentioned, binding materials utilized in the recording media are required to have both an excellent dispersing ability and a strong binding force simultaneously to disperse the very fine powder homogeneously and bind it tightly in the media.
Binding materials which are generally utilized for recording media are polyester resins; cellulosic resins; polyurethane resins; phenolic resins; epoxy resins; polyamide resins; polymers and copolymers of vinyl chloride, vinylidene chloride, vinyl acetate, (meth)acrylic esters, (meth)acrylamide, acrylonitrile, butadiene, styrene, vinyl esters and the like monomers; and the like materials used in combination with so called dispersing agents, such as low molecular weight surface active agents and fatty acids. However, among these generally utilized binding materials, no material satisfies the requirement that fine powder of magnetic materials must be dispersed homogeneously and, at the same time, the magnetic layer prepared must have excellent wear resistance.
Improvement of dispersion of powder of magnetic materials in binding materials has been a subject of study. It was reported that, when powder of a magnetic material is treated with aminosilane compounds, the dispersion is improved (Laid Open Japanese Patent Publication Showa 58-205929). It was also reported that, when a ternary copolymer of vinyl chloride, vinyl alcohol and vinyl acetate which is generally utilized as the binding material is modified with aminosilane compounds and used as the binding material, the dispersion is improved (Japanese Patent Publication Heisei 1-49426). However, the magnetic coating materials prepared according to these reports do not have good enough processability which is required for application of the coating material to the surface of base materials.
It is expected that an increase of the strength of the magnetic layer of a recording medium will enhance the durability of the recording medium. A binding material with increased strength is necessary for preparing the magnetic layer of increased strength. The binding material must be, at the same time, capable of forming a highly filled magnetic layer by homogeneously dispersing fine powder of magnetic materials. Increase of molecular weight of the binding material is effective for the increase of the strength of the binding material. However, the increase of molecular weight of the binding material causes an increase of viscosity of the coating material in which the powder of magnetic material is dispersed and the dispersing ability tends to become poorer. Furthermore, the finer the powder of the magnetic material, the more the viscosity of the coating material is increased. Because of the reason described here, methods to increase strength of binding material without increasing the molecular weight of the material are required. Crosslinking of the binding material by using polyisocyanate compounds is generally used as one of the methods. To crosslink the binding material by using polyisocyanate compounds, the binding material must comprise a structure which can react with the polyisocyanate compounds. It is generally practiced that groups having active hydrogen which can react with polyisocyanate compounds, such as the hydroxy group, are introduced into the binding material.
However, when a large amount of the hydroxy group is introduced into the binding material and the coating material contains lubricants and other like ingredients, such as fatty acids and fatty acid esters, stability of the dispersion of the powder of magnetic material is decreased and stable and reliable preparation becomes difficult. On the other hand, when the amount of the hydroxy group present in the binding material is less than the necessary amount, the degree of crosslinking is insufficient and the improvement of the strength can not be attained. Furthermore, the excess amount of isocyanate group of the polyisocyanate compound which exists more than adequate for the reaction with the hydroxy group reacts with moisture in the air and the magnetic layer formed becomes fragile. The method of crosslinking by using polyisocyanate compounds is not suitable for preparing reliable recording media with enough stability.